The present invention relates to wiped-film evaporators, and especially to such evaporators for use with corrosive liquids.
In one known form of wiped-film evaporator a thin film of liquid to be evaporated is formed on the internal wall of an upright cylindrical evaporation vessel by means of wiping elements carried by a rotary vertical shaft in the vessel, and heating means cause evaporation of the film. Such an evaporator is shown in British Pat. No. 1288431 (Henry Balfour & Co. Limited). The wiping elements provide a rapidly changing film by fluid friction, and good heat transfer rates are maintained over the entire length of the evaporator because the wiper elements effectively wipe the entire heating surface irrespective of the film thickness. Dry spots and channelling are eliminated and nucleate (bubble) boiling is substantially reduced thus minimising high localised heat fluxes into the process liquor which, with heat sensitive chemicals and foodstuffs, can be detrimental to quality, taste and flavour. The wiped film evaporator can be used for evaporation of organic compounds, concentration of aqueous solutions, solvent recovery, stripping of aqueous solutions, and concentration of viscous products.
To enable the evaporator to handle corrosive liquids, it has been customary to use alloys, such as for example stainless steel, for parts of the evaporator coming into contact with the corrosive liquids and/or products of the process. Such parts include the vessel, shaft and wiping element support means: the wiping elements themselves can be made of a plastics material. However, the manufacture of parts from alloys is relatively expensive and further alloy parts are not completely satisfactory for some processes, for example those operating at higher temperature, those with higher purity requirements and those dealing with more corrosive liquid or products. In particular, the alloy parts could contaminate the product, and cleaning may be difficult.
Additionally, it has been customary to have the bottom bearing for the shaft located within the evaporator vessel, and supported from the vessel wall for example by means of radial struts. For processes involving corrosive liquids, the life of the bottom bearing will be greatly reduced, and although removable cartridge type bearings have been employed to facilitate bearing maintenance or replacement, this complicates the evaporator construction.